Russian President Vladimir Putin just held a news conference. He said he won't send troops into Ukraine except, quote, "under proper procedures." Understanding Russia's actions and intentions have been a challenge. Putin spoke as though there are no Russian soldiers in the Crimea region. After seizing a border crossing, Russian troops actually began pouring in by ferry late yesterday and took control.

Another force of tens of thousands of troops had massed for military exercises within Russia close to the border with Ukraine. Putin is sending those troops back to their bases, saying the exercises are complete.

Today, NATO is holding emergency consultations on the situation. Admiral James Stavridis once served as the NATO Supreme Allied Commander for Europe. He's now dean of The Fletcher School of Law and Diplomacy at Tufts University, and joins me on the line.

Admiral, welcome to the program.

JAMES STAVRIDIS: Thanks. Great to be on.

GREENE: Let's start with one fact. Ukraine is not a member of NATO but the country does have a partnership agreement with the alliance. Given that, what is NATO's obligation here, if any?

STAVRIDIS: NATO has no treaty obligation to Ukraine. It's worth noting that Ukraine has participated with NATO in the missions in Afghanistan, the Balkans, was extremely supportive during the Libyan campaign, counter-piracy operations off the coast of Africa. So, Ukraine has been a strong partner to NATO. However, the NATO treaty would not require a military response.

GREENE: No one is talking at this point in NATO countries about some sort of military response to actually face down Russian forces in Crimea and Ukraine. But are there other ways that NATO can provide some sort of support here?

STAVRIDIS: I think so. I think it's conceivable and, of course, these would be political decisions that the North Atlantic Council would have to take - the governing body of NATO. But they could include things like intelligence hearing, logistic support, advising. So, there are mechanisms for support short of military action, which nobody wants to see at this point.

GREENE: Well, you talk about sort of the sharing information and military cooperation. The U.S. is now suspending not only trade talks, but also military exercises with Russia to send some sort of message. Is that effective? I mean is that likely to get Putin's attention?

STAVRIDIS: I don't think it will. Although, it, every little bit will help so I think the increase of political, economic and other factors taken together, I think over time can have some effect. But an individual set of canceling exercises probably won't change Putin's calculus.

GREENE: Well, Admiral, when it comes to NATO, you have called for NATO to do some contingency planning in case of a full-scale Russian invasion of this country beyond Crimea. What exactly are you suggesting NATO do?

STAVRIDIS: We do a fair amount of planning. And I think that in a situation like this it would be limited to the kinds of things I've already addressed. So, intelligence sharing, information sharing, logistic support. I don't foresee contingency planning that would anticipate combat operations.

GREENE: Can I just ask you to take me and us into the minds of people who are in the Ukrainian military? I mean I'm sure that they realize firing a shot or doing something in defense of their country could really exacerbate a situation. But, of course, they're seeing their country sort of under siege in a way. I mean what sort of situation and mindset do you think they're in right now?

STAVRIDIS: I think they're probably of two minds, and you've categorized it pretty well. On the one hand, they don't want an escalation of the situation, they don't want a shooting war with Russia. They would be seriously outgunned by the Russian military, obviously. On the other hand, like anybody in their own country, people in the military are proud. They have a military tradition. They're a capable military, although, not a large one or a technologically advanced one. So within their bases, evidently surrounded by Russian troops, they must feel a sense of shame and helplessness in the face of an adversary who has invaded their country and surrounded them in their own bases.

GREENE: All right. Admiral James Stavridis is dean of The Fletcher School of Law and Diplomacy at Tufts University and former NATO Supreme Allied Commander for Europe. Admiral, thank you.